[PATCH 4/4] x86/entry/64: Bypass enter_from_user_mode on non-context-tracking boots

[Andy Lutomirski]

On CONFIG_CONTEXT_TRACKING kernels that have context tracking
disabled at runtime (which includes most distro kernels), we still
have the overhead of a call to enter_from_user_mode in interrupt and
exception entries.

If jump labels are available, this uses the jump label
infrastructure to skip the call.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/entry/calling.h  | 15 +++++++++++++++
 arch/x86/entry/entry_64.S |  8 ++------
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3c71dd947c7b..271e30c585bc 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -1,3 +1,5 @@
+#include <linux/jump_label.h>
+
 /*
 
  x86 function call convention, 64-bit:
@@ -232,3 +234,16 @@ For 32-bit we have the following conventions - kernel is built with
 
 #endif /* CONFIG_X86_64 */
 
+/*
+ * This does 'call enter_from_user_mode' unless we can avoid it based on
+ * kernel config or using the static jump infrastructure.
+ */
+.macro CALL_ENTER_FROM_USER_MODE
+#ifdef CONFIG_CONTEXT_TRACKING
+#ifdef HAVE_JUMP_LABEL
+	STATIC_JUMP_IF_FALSE .Lafter_call_\@, context_tracking_enabled, def=0
+#endif
+	call enter_from_user_mode
+.Lafter_call_\@:
+#endif
+.endm
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f585df24ab3d..9b49b56efa29 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -517,9 +517,7 @@ END(irq_entries_start)
 	 */
 	TRACE_IRQS_OFF
 
-#ifdef CONFIG_CONTEXT_TRACKING
-	call enter_from_user_mode
-#endif
+	CALL_ENTER_FROM_USER_MODE
 
 1:
 	/*
@@ -1058,9 +1056,7 @@ ENTRY(error_entry)
 
 .Lerror_entry_from_usermode_after_swapgs:
 	TRACE_IRQS_OFF
-#ifdef CONFIG_CONTEXT_TRACKING
-	call enter_from_user_mode
-#endif
+	CALL_ENTER_FROM_USER_MODE
 	ret
 
 .Lerror_entry_done:
-- 
2.4.3

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